Ball screws used in machine tools or automobiles actuators, are rotationally supported by rolling bearings, such as deep groove ball bearings, at both ends of the screw shaft. In general, an inner ring of the rolling bearing, to which an axial load of the screw shaft is applied, is immovably axially secured by a snap ring. This provides inexpensive and easy assembly.
However, it is inevitable when using a snap ring that an axial looseness caused by dimensional errors between the ring groove and the inner ring of the bearing will occur. Thus, this often causes problems in reliability and durability of the ball screw due to slipping off or damage of the snap ring and ultimately the inner ring coming out of the bearing.
To solve such a problem, a supporting structure for a ball screw shaft is known and shown in FIG. 4. The end of the ball screw shaft 51 is formed with a fitting portion 52 and a shoulder 57. An end face of an inner ring 56 of a rolling bearing 53 abuts the shoulder 57. The end face of the shoulder 57 of the ball screw shaft 51 is formed by tops and bottoms of the screws and threads and thus the end face of the shoulder 57 is not formed by a perfect flat annular surface. Accordingly, the outer diameter of the fitting portion 52 is formed smaller than the bottom diameter of the screw grooves (not shown). The end of the ball screw shaft 51 is formed with an annular groove 54 having a side wall 54a inclined at a predetermined angle α. A securing ring 55 is snapped into the annular groove 54 between the side wall 54a and a chamfered surface 56a of the inner ring 56 of the rolling bearing 53.
The securing ring 55 is formed by a wire having a circular cross section. Its inner diameter is somewhat smaller than the outer diameter of the fitting portion 52 of the screw shaft 51. The diameter of the wire itself is somewhat larger than an annular gap formed between the chamfered surface 56a and the side wall 54a of the annular groove 54. The securing ring 55 is resiliently snapped in the annular groove 54 between the chamfered surface 56a and the annular groove 54. Thus, the inner ring 56 is urged forward to the end face of the shoulder 57 of the ball screw shaft 51 by an axial force as a component force by the action of the inclination angle α of the side wall 54a. Thus, the inner ring 56 is secured on the fitting portion 52 of the ball screw shaft 51 without any axial looseness. Accordingly, it is possible to reduce assembly steps, the number of parts and thus the manufacturing cost. This produces a realized reduction of size of the supporting portion as well as improves the reliability and durability of the ball screw (see Japanese Laid-open Patent Publication No. 233358/2005).